The search for more effective chemotherapeutic strategies has resulted in the identification of novel compounds that can overcome the resistance of tumors to cell death; however, these agents are limited in that most are ineffective in the treatment of solid tumors. PS-341, which is a unique inhibitor of the proteasome, has anticancer activity against myeloma cells but does not kill solid tumors. Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) induces apoptosis by ligation of specific receptors; however, most solid tumor cell lines are moderately to completely resistant to this agent. In an attempt to enhance the ability of these agents to kill solid tumors we combined them in non-toxic doses in tissue culture. We observed an extremely high degree of synergism between these agents in their ability to induce apoptosis of tumor cells, including prostate, bladder and colon cancer cell lines, which are resistant to either agent when used individually. Notably, this synergistic apoptosis effect occurs in the presence of expression of resistance genes, e.g., bcl-xL, and in the absence of caspase-9 and Bax. Our preliminary data indicate that the synergism is associated with specific biochemical changes that are induced by treatment with PS-341 treatment. We have shown that the synergistic killing effect requires the presence of Bak protein and that the effects of PS-341 may be secondary to the ability of this compound to elevate the levels of the Dr4 and Dr5 TRAIL receptors, and to increase the levels of the proapoptotic protein, Bim. The goal of this proposal is to investigate the mechanism of action behind this impressive synergy by identifying the mechanisms by which PS-341 regulates: (1) The amount of TRAIL receptors; (2) The activity of the Bak protein; and (3) The levels of Bim. Finally, animal models will be used to evaluate this combination in live hosts. Information generated will facilitate the development of this combined anticancer therapy and shed light on the mechanisms by which solid tumors evade apoptosis thus enabling the design of the next generation of chemotherapeutic agents.